rspec/rules/S6229/cfamily/rule.adoc

The ``++chrono++`` library, introduced in {cpp}20, provides support for calendars, time zones, and i/o formatting and parsing operations on time-related objects. 


``++chrono++`` is a better alternative to the C/POSIX functions that operate on ``++time_t++``, ``++tm++``, or ``++timespec++`` types. In comparison to C facilities, it provides a better integration with other components of the {cpp} standard library: (``++iostreams++`` and ``++format++``). Also, it supports compile-time computation and it is thread safe.


This rule raises an issue on any use of C/POSIX functions that can be replaced with one of the ``++std::chrono++`` components:

* querying for current time (``++time++``, ``++timespec_get++``, ``++clock_gettime++``)
* date to time-point conversion (``++mktime++``, ``++gmtime++``, ``++localtime++``)
* time serialization (``++ctime++``, ``++asctime++``, ``++strftime++``)
* time parsing (``++strptime++``)

== Noncompliant Code Example

----
int currentMonth() {
  std::time_t tp;
  std::time(&tp);
  std::tm* date = std::gmtime(&tp);
  return date->tm_mon + 1;
}

std::chrono::system_clock::time_point makeSomeDay() {
  // Creates time_point corresponding to 2020-09-04
  std::tm date{};
  date.tm_year = 120;
  date.tm_mon = 8;
  date.tm_mday = 4;
  std::time_t t = std::mktime(&date); // Noncompliant
  return std::chrono::system_clock::from_time_t(t);
}

std::optional<int> yearOfTimePoint(std::chrono::system_clock::time_point tp) {
  std::time_t t = std::chrono::system_clock::to_time_t(tp);
  std::tm* date = std::gmtime(&t); // Noncompliant
  if (!date)
    return std::nullopt;
  return date->tm_year + 1900;
}

std::string toIsoString(std::chrono::system_clock::time_point tp) {
  std::time_t t = std::chrono::system_clock::to_time_t(tp);
  std::tm* date = std::gmtime(&t);  // Noncompliant
  if (!date)
    throw InvalidDate();

  std::string buffer(100, ' ');
  std::size_t written = std::strftime(buffer.data(), buffer.size(), "%F", date);
  buffer.resize(written);
  return buffer;
}
----

== Compliant Solution

----
std::chrono::month currentMonth() {
  using namespace std::chrono;
  auto dp = floor<days>(system_clock::now());
  return year_month_day(dp).month();
}

std::chrono::system_clock::time_point makeSomeDay() {
  using namespace std::chrono;
  return sys_days(2020y/September/4);
}

std::optional<std::chrono::year> yearOfTimePoint(std::chrono::system_clock::time_point tp) {
  using namespace std::chrono;
  year_month_day date(floor<days>(tp));
  if (!date.ok())
    return std::nullopt;
  return date.year();
}

std::string toIsoString(std::chrono::system_clock::time_point tp) {
  return std::format("{:%F}", tp);
}
----